Here is the basic Dashi recipe, which can be used for most Japanese dishes. It can be kept in the refrigerator for 1 week or in the freezer for up to a month.
Ingredients (about 3 cups):
Place water and konbu kelp in a saucepan over medium heat.
Just before the water boils, remove the kelp from the pan. Increase the heat to high and bring to a boil and keep cooking for 5 minutes.
Add 1/4 cup of room temperature water (not in the ingredients) to the pan to cool it down a bit. Then add bonito flakes into the pan.
When the water reaches a boil, turn heat off.
After 5 minutes off of the heat, strain the liquid through a strainer lined with a paper towel or several layers of cheesecloth. Refrigerate if using later.